28-30mm works well on the GY6. Even an intake, exhaust or cam change may increase the need the for additional flow, so don't think you have to do major top or bottom end work to benefit from a 15-20% larger carburetor.
I prefer mechanical slide carburetors, but the jetting is more sensitive to temperature and elevation changes. If you're willing to rejet based on conditions or you ride in similar conditions most of the time, this is not an issue.
|